1. Field of the Invention
This invention relates to a method of operating an electrophoretic indicating element and to an apparatus which includes such an element and is arranged to operate the element according to this method.
2. Description of the Prior Art
The operation of electrophoretic indicating elements has been described in detail by I. Ota, J. Ohnishi and M. Yoshiyama (Proc. IEEE, Vol. 61 (1973) pages 832-836). When such indicating elements are used, agglomerations of the pigment particles are observed after some time, for example in the form of rasters. The individual raster dots consist of a plurality of particles of which only some, namely those at the periphery, and after prolonged operation not even these, switch over, that is to say migrate to the other electrode on a reversal of the external electrical field. As a result of the clearly visible reticulation (the distance between the individual raster dots is between 0.5 and 2 mm for example) or other agglomerations, the general impression of the indication is very adversely affected and the contrast between surface elements with different switching states is considerable reduced.
It has already been proposed by manufacturers of such electrophoretic indicating elements that, on the appearance of visible agglomerations of the pigment particles, the indicating elements should be operated for a while with a rectangular alternating voltage of 1 Hz and 150 V.sub.ss for example. Such a method requires an interruption in the indicating operation, however. In addition, the disturbing raster formation is only eliminated for a relatively short period of time.